1. Field of the Invention
This invention relates to a laser beam scanning apparatus, and more particularly to a laser beam scanning apparatus for scanning an object by using a plurality of laser beams each having a different wavelength from the others, photodetection and photoelectrical conversion of the reflected or transmitted laser beams being then employed to produce image data of the object, and to a laser beam scanning apparatus for scanning an object by using a plurality of laser beams, each having a different wavelength, intensity-modulating the laser beams by a corresponding video signal and scanning the medium to produce a color image thereon.
2. Description of the Prior Art
The flying spot scanning-type video input system by means of which an object is scanned by a laser beam spot and the light reflected or transmitted therefrom is picked up by a photosensor to obtain video signals and image output systems whereby a spot of laser light that is intensity-modulated by video signals is used to scan a medium, such as a screen or film to display or record an image thereon feature a number of advantages, such as the brightness, convergence and coherence of the laser beam. These systems are widely used in industrial and medical fields. Using the laser beam to scan two-dimensionally, horizontally and vertically, and having the scanning rate correspond to the raster scan of an ordinary TV system enables a real-time video image to be obtained which is free of residual images and, as such, has a broader range of use and markedly improved operability. See for example U.S. Pat. No. 4,213,678 and Japanese Laid-open Patent Application No. 62(1987)-117524.
In such a system, means for deflecting the laser beam to scan horizontally or vertically include mechanically driven methods that employ a swinging mirror or a polygonal mirror or other such rotating, multi-faceted mirrors, and non-mechanical methods such as acousto-optical deflectors and the like. With an acousto-optical method, tracking is simplified since it can be carried out at the horizontal scanning frequency of 15.75 KHz in the NTSC system. Also, the deflector is small, and stable contains few parts that wear out due to mechanical of operation. Such a system is highly reliable and long-lasting.
However, because acousto-optical deflectors employ diffraction, they give rise to color dispersion from first-order diffraction when using a plurality of laser beams each having a different wavelength.
FIG. 3 shows the operation of an acousto-optical deflector 60 that is driven by a signal source 61. If the ultrasonic driving frequency is f, the wavelength of the incident laser beam is .lambda. and the ultrasonic velocity is v, the angle of diffraction .theta. of the first-order diffraction obtained by the operation of the acousto-optical deflector is: EQU .theta..apprxeq..lambda.f/v
However, with such a deflection system, unlike a mechanically driven mirror-type method, because the angle of diffraction is dependent on the wavelength of the incident beam, color dispersion occurs. For example, when the three-color R (red), G (green), and B (blue) laser beam shown in FIG. 4 impinges on the acousto-optical deflector 60, the longer wavelengths produce larger angles of diffraction, so first-order diffraction color dispersion is produced. Therefore, when a laser source which produces a plurality of laser beams is used to obtain chromatic information about an object, the drawback is that the coloring of the images has been difficult.
The present applicants previously developed a technique for compensating for the effects of color dispersion caused by the deflector. This technique consisted of the provision of electronic processing means to process the signal using a different time-base for each wavelength corresponding to the color dispersion produced by the deflection means (c.f. Japanese Patent Applications Nos. 61(1986)-80236 and 61(1986)-80237 corresponding to U.S. patent applications Ser. Nos. 35,091 and 35,090 filed on Apr. 6, 1987).
The above technique is illustrated by FIG. 5. The effect of deflector-induced color dispersion is removed electronically by varying the timing during each scanning period of the storage in, and retrieval from memory of predetermined signals corresponding to the R, G and B wavelengths. (For further details please refer to the specifications of the above applications.) This technique offered a low-cost way of removing chromatic shifts arising from color dispersion that is an inherent drawback of the acousto-optical deflector. It also enables the realization of high scanning frequencies. Also, the deflector can be made compact and stable, for a long, durable working life and high reliability.
However, when using wavelengths of the three primary colors, R, G and B, this electronic signal processing means has to determine six constants in all, consisting of the clock frequency and the start-timing of the memory write-in or read-out for each wavelength. The adjustment is thus time-consuming. Moreover, the temperature of the circuitry has to be controlled, especially that of the oscillators, to prevent external temperature fluctuations from producing variations in clock frequencies. Otherwise, even after adjustment to exclude the effects of color dispersion, in some cases chromatic shift was produced. Namely, in the said applications the signals used to control the timing of the memory write-in or read-out operations are produced as synchronized signals, mainly based on horizontal synchronization signals, so it has been time-consuming to perfectly adjust the delay time from the synchronization signals, the frequency of the oscillator and the like. Even after these adjustments there was still a possibility that, unless strict control of the temperature was exercised, fluctuations might still arise after the adjustment process.